Recording and reproduction of sound



Oct. 24, 1939. K. scHwARZ RECORDING AND REPRODUCTION OF SOUND I Filed July 23, 1936 INVEN'I'OR KarL Schwarz BY;

ATTOR EY Patented Oct. 24, 1939 PATENT OFFICE 2,177,081 RECORDING'AND REPRODUCTION OF SOUND I ,Karl .Schwarz, Berlin,

Germany, assignor t Klangfilm G. m. b. H., Berlin, Germany, a corporation of Germany Application July 23, 1936,Serial-No. 92,084 In Germany October 30, 1935 1 Claim.

This invention relates to the recording and reproduction of electrical impulses, such as those of audio frequency, and has for its principal ob ject the provision of an improved apparatus and 1 method of operation for minimizing the undesired noise resulting'from the imperfect resolving power of photographic emulsions in connection with high frequency impulse records.

As pointed out in a copending application of Glenn L. Dimmick, Serial No. 610,301, filed May 9, 1932 now Patents No. 2,074,049, granted March 16,.1937,it is well known that objectionable noise, commonly known as ground noise, is produced by scratches and dust on the clear part of a sound record. Various means have been provided for eliminating such distortion. In the variable area type of recording they assume the form of a shutter which is caused to follow the envelope of the sound record and toreduce the clear portion of the record to the minimum required for proper reproduction of sound. In the variable density type of recording they assume the form of a biased light valve. Neither of these forms, however, have satisfactorily solved the problem of eliminating the undesirable effects of decrease in the resolving power of the record material as the frequency of the recorded sound increases. This effect is particularly disagreeable in the case of small records such as are used in connection with 16 millimeter recordings,

In the reproduction of a talking film on which there is recorded high frequency impulses of comparatively large volume, an undesirable noise is often produced.

This undesirable noise is caused by imperfections of the film emulsion. If sound oscillations of high frequency are being recorded, it may happen that during the development the places between the exposed and unexposed parts of the emulsion become blurred. This may occur in the case of variable area recording as well as variable density recording. In the case of variable area films, the eifect appears to be greater than in the case of variable density films. In both cases the record tends to become blurred at high frequencies. This is synonymous with a partial detection of the record over several cycles of the sound oscillation. Hence, there appears at these places a detected component superposed on the sound oscillations of high frequency. This detected component has the effect of an oscillation which is still within the audible limit of the ear and operates to produce the undesired noise effect.

The method which applicants apply to eliminate this undesired effect resides in that the high (01. 179-1003) sound frequencies by which the direct current (which causes the effect) is produced, are detected, and that-this detected current is so regulated as to be useful for compensating the D. C. causing the effect;

This invention may also be utilized in recordings which at high frequencies likewise do not permit of a complete development of the fine peaks of the record 1. e. the latter are cut off. This likewise signifies a detection of the high frequency recorded. As distinct from the effect in which the direct current produced owing-to the blurred state of the record, will be added to the direct; current of the steady light, in the aforementioned case, the direct current produced would be subtracted from the direct current of the steady light.

Since detection increases with higher frequencies, a feature of the invention resides in causing the detection for the compensation of the effect, likewise to increase with an increase in frequency.

The invention will now be elucidated in connection with the circuit shown.

The figure represents an example of a circuit arrangement for compensating the undesired noise effect. The current variations originated in the photo-cell are conducted to the terminal I of the input circuit of tube 2 either directly or across one or several amplifiers. The current variations are amplified in tube 2 in a known manner, and are reproduced in the loudspeaker 3. Obviously, tube 2 need not necessarily be the last tube of the reproducing amplifier since several amplifier tube could still be placed in series.

The undesired noise effect is compensated or neutralized in the following manner: In the anode circuit of tube 2, at a point 4 there is inserted a variable condenser 5 connected to ground across a transformer 6. The secondary of the transformer is tapped in the center and is likewise connected to ground, across a grid biasing battery I. The two outer terminals of the secondary winding of transformer 6 are connected to the resistor I0, across detectors or rectifiers 8, 9. The resistor I0 is provided with a slide I I for the tapping of different voltages. The detectors 8, 9 are so connected that current can flow only in one direction through the resistor II] to the adjustable terminal II of this resistor, for example, according to the arrangement shown in the drawing, in the direction from detector 8 towards detector 9. From the slide I I of resistor I0 there is provided a connection to the grid of tube 2 across resistor I2 and resistor I3. The condensers I4, I5 both placed at ground on the one hand, and on the other hand, each connected respectively to a side of the resistor l2, simply serve for smoothening the current detected by the detectors 8 and 9.

If a sound oscillation, superposed on a detected current portion which produces the undesired noise effect, arrives at terminal I, a part of the amplified portion of the sound frequency will be passed across condenser and transformer 6, and produces, due to detection, a voltage drop through resistor l0. sition of the slide II, this detected component can be passed to tube 2 as a lower or higher biasing potential. Now, the slide must be so set, that the detected current has exactly the same value as the do. portion of. the oscillation-producing the undesired efiect, whereby however the sign of the former is opposite to that of the latter.

Through variation of condenser 5 the frequency can be adjusted to that which detection is to begin in 8 and 9. Since the resistance of the condenser 5 decreases with higher frequencies, there occurs automatically at H a, greater detection at higher frequencies. Since however, the undesired effect likewise increases at higher frequencies, an automatic compensation can be carried out in 1 this simple way. It is only necessary, so to set 'the condenser 5 and the slide H for a certain film, that at the lowest tone frequency atjwhich.

quency (increase in undesired noise effect) the circuit automatically takes care of the compensation. V

In accordance with the po-- If detection occurs due to the fact that the peaks of the high frequencies are no longer sufficiently exposed and thus cut off in the copy, it is necessary in order to compensate this direct current, that the slide ll be displaced in the direction opposite to that required for compensating the undesired noise efiect. If a film is to be reproduced in which no undesired effect occurs, the slide is to be placed in the center of resistor H]. In this case the voltage at resistor I0. produced by detection of the tone frequencies is equal to zero. 7

It should also be pointed out that without departing from the spirit of the invention, instead of using a backward control of the grid bias as shown inthe figure, a forward control 7 By the term forquency range, and means connected between said circuits including a condenser, a transformer en,

ergized through said condenser, detector means. 

